Abstract: Systems and methods for controlling a vehicle's speed. The methods comprise: obtaining sensor data by an Aftermarket Electronic Controller (“AEC”) that was installed in the vehicle after the vehicle's sale to a consumer; processing by AEC the sensor data to determine if there is a Collision Risk (“CR”) of the vehicle colliding with a Possible Obstruction (“PO”); calculating by AEC an estimated time until PO and the vehicle will collide, when a determination is made that there is a CR; performing first operations by the AEC to transition the vehicle's operational state to a collision warning state in which a warning device is activated, when the estimate time is greater than a threshold value; and performing second operations by AEC to transition the vehicle's operational state to a collision avoidance state in which a speed retarder is engaged, when the estimated time is less than the threshold value.